Nine chlorine plants in the South and East pour at least eight tons of mercury into the environment each year _ a situation that demands federal action to force companies to convert to cleaner technology, activists said Wednesday.

Environmentalists think the amount of mercury emitted by the plants may be even greater; the industry acknowledges that tons of the toxic metal are unaccounted for each year, though it does not believe that mercury is dumped into the environment.

Chlorine at the plants is made by pumping electrically charged salty water through a vat of mercury, a process devised more than 100 years ago. Environmentalists say these plants are a largely ignored and unchecked source of mercury pollution.

Mercury settles in waterways and accumulates in fish. In humans who eat those fish, the metal can cause neurological and developmental problems, particularly in fetuses and children.

Citing chlorine factories as a "major global source of mercury," the Washington, D.C.-based environmental group Oceana on Wednesday called on the Environmental Protection Agency to require all the plants to convert to mercury-free technology by 2008.

Arthur Duncan, vice president of safety and health for The Chlorine Institute, a trade group based in Arlington, Va., said emissions have been significantly reduced in the past decade.

"Certainly mercury has been a concern for a long time to people and it's an environmental issue that we're working to address," he said.

The calculations of how much mercury is dispersed into the environment are in dispute. The industry, in reports to the EPA, says eight tons _ about three 50-gallon barrels _ were emitted in 2003.

For example, the companies said a plant in Muscle Shoals, Ala., emitted 1,757 pounds of mercury that year; another, in New Castle, Del., released 2,863 pounds.

But the environmentalists say these calculations may be wrong, because while the companies monitor the amount of mercury that goes out of their smokestacks, they merely estimate the amount that evaporates and leaves the factories through vents.

In addition, industry officials acknowledge that they cannot acount for an additional 30 tons a year. They say that it could be stuck in factory pipes, and they are trying to find it.

The environmentalists are skeptical. They think even more mercury is missing, pointing out that more mercury is delivered to the plants each year than is going out _ 65 tons more in 2000 alone, said Oceana's Jacqueline Savitz, co-author of the report.

But industry officials say that mercury purchases do not necessarily equal mercury use, because some of it is simply stored to be used later.

EPA spokeswoman Cynthia Bergman said the question of where all this mercury went is very important to her agency, but that the EPA's "best information indicates that the mercury is not being emitted into the air."

While total mercury emissions in the United States have fallen substantially since 1990, power plants remain the largest remaining human-caused source. They released 90,370 pounds of mercury into the air in 2002, the most recent year for which EPA data are available.

Federal guidelines released last February place strict limits on the amount of mercury that power plants can release.

They place no similar caps on chlorine plants, but do require more frequent emission measurements and equipment inspections, "significantly more stringent requirements" than had been in force before, said Vito Fiore, a vice president of Vulcan Chemicals, which has a plant in Port Edwards, Wis.

Last year, three environmental groups _ Earthjustice, Natural Resources Defense Council and Sierra Club _ sued the EPA and began using other legal channels to get power plant emission limits extended to chlorine plants.

The other plants are in Charleston, Tenn.; New Martinsville, W.Va.; Ashtabula, Ohio; Lake Charles, La.; St. Gabriel, La., and Augusta, Ga.